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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="lockfree.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
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<div class="toc"><dl class="toc">
<dt><span class="section"><a href="rationale.html#lockfree.rationale.memory_management">Memory Management</a></span></dt>
<dt><span class="section"><a href="rationale.html#lockfree.rationale.aba_prevention">ABA Prevention</a></span></dt>
<dt><span class="section"><a href="rationale.html#lockfree.rationale.interprocess_support">Interprocess
      Support</a></span></dt>
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<div class="titlepage"><div><div><h3 class="title">
<a name="lockfree.rationale.memory_management"></a><a class="link" href="rationale.html#lockfree.rationale.memory_management" title="Memory Management">Memory Management</a>
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<p>
        The lock-free <code class="computeroutput"><a class="link" href="../doxygen/classboost_1_1lockfree_1_1queue.html" title="Class template queue">boost::lockfree::queue</a></code>
        and <code class="computeroutput"><a class="link" href="../doxygen/classboost_1_1lockfree_1_1stack.html" title="Class template stack">boost::lockfree::stack</a></code>
        classes are node-based data structures, based on a linked list. Memory management
        of lock-free data structures is a non-trivial problem, because we need to
        avoid that one thread frees an internal node, while another thread still
        uses it. <code class="literal">boost.lockfree</code> uses a simple approach not returning
        any memory to the operating system. Instead they maintain a <span class="bold"><strong>free-list</strong></span>
        in order to reuse them later. This is done for two reasons: first, depending
        on the implementation of the memory allocator freeing the memory may block
        (so the implementation would not be lock-free anymore), and second, most
        memory reclamation algorithms are patented.
      </p>
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<div class="titlepage"><div><div><h3 class="title">
<a name="lockfree.rationale.aba_prevention"></a><a class="link" href="rationale.html#lockfree.rationale.aba_prevention" title="ABA Prevention">ABA Prevention</a>
</h3></div></div></div>
<p>
        The ABA problem is a common problem when implementing lock-free data structures.
        The problem occurs when updating an atomic variable using a <code class="literal">compare_exchange</code>
        operation: if the value A was read, thread 1 changes it to say C and tries
        to update the variable, it uses <code class="literal">compare_exchange</code> to write
        C, only if the current value is A. This might be a problem if in the meanwhile
        thread 2 changes the value from A to B and back to A, because thread 1 does
        not observe the change of the state. The common way to avoid the ABA problem
        is to associate a version counter with the value and change both atomically.
      </p>
<p>
        <code class="literal">boost.lockfree</code> uses a <code class="literal">tagged_ptr</code> helper
        class which associates a pointer with an integer tag. This usually requires
        a double-width <code class="literal">compare_exchange</code>, which is not available
        on all platforms. IA32 did not provide the <code class="literal">cmpxchg8b</code> opcode
        before the pentium processor and it is also lacking on many RISC architectures
        like PPC. Early X86-64 processors also did not provide a <code class="literal">cmpxchg16b</code>
        instruction. On 64bit platforms one can work around this issue, because often
        not the full 64bit address space is used. On X86_64 for example, only 48bit
        are used for the address, so we can use the remaining 16bit for the ABA prevention
        tag. For details please consult the implementation of the <code class="literal">boost::lockfree::detail::tagged_ptr</code>
        class.
      </p>
<p>
        For lock-free operations on 32bit platforms without double-width <code class="literal">compare_exchange</code>,
        we support a third approach: by using a fixed-sized array to store the internal
        nodes we can avoid the use of 32bit pointers, but instead 16bit indices into
        the array are sufficient. However this is only possible for fixed-sized data
        structures, that have an upper bound of internal nodes.
      </p>
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<div class="titlepage"><div><div><h3 class="title">
<a name="lockfree.rationale.interprocess_support"></a><a class="link" href="rationale.html#lockfree.rationale.interprocess_support" title="Interprocess Support">Interprocess
      Support</a>
</h3></div></div></div>
<p>
        The <code class="literal">boost.lockfree</code> data structures have basic support
        for <a href="../../../libs/interprocess/index.html" target="_top">Boost.Interprocess</a>.
        The only problem is the blocking emulation of lock-free atomics, which in
        the current implementation is not guaranteed to be interprocess-safe.
      </p>
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<div class="copyright-footer">Copyright © 2008-2011 Tim
      Blechmann<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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